Fertilization is the first stage of human development according to thebasics of embryology . This process begins when a sperm unites  with the mature ovum and it ends with the creation of the zygote . It normally occurs in the lateral third ( ampullary) part  of the uterine tube, the widest part that lies close to the ovary, and usually fertilization process takes about 24 hours...

The human sperms move at a speed of about 3 mm / mm through the female genital tract and reach the uterine tubes 30-60 minutes after ejaculation .  Not all sperms deposited in the vagina reach the cervix , only 1% of them can do so and  may survive for many hours  i.e. of the 200 to 300 million sperms deposited , only 300 to 500 reach the fertilization site.

The sperms migrate this distance  depending mainly on their own active propulsive motility that may be helped by fluid movements created by uterine cilia.

Sperm binding :

At the tube isthmus , the sperms stop migration as they become less motile , but upon ovulation they regain motility , probably due to production of chemo-attractants by the cells of the cumulus oophrous surrounding the ovum, and  move towards the ampullary part of the tube where fertilization usually occurs.  Numerous sperms reach the surface of the oocyte and try to penetrate its coverings. However , sperms are unable to fertilize ova until they are capacitated .

Capacitation:

This is the final step in maturation of sperms that is required to render them competent to fertilize an oocyte. It is a biochemical event ; the sperm moves normally and look mature prior to capacitation.  It typically occurs after ejaculation, in the female reproductive tract. It lasts about 7 hours  in humans and takes place mainly in the uterine tubes through interaction between their epithelial lining and  the sperms .This  process removes a glycoprotein coat and  seminal plasma proteins from the plasma membrane overlying the acrosomal region of the sperm.

Phases of fertilization:

Fertilization  passes through the following phases:

Phase I ( penetration of the corona radiata ):

A number of sperms pass through the cells of the corona radiata to  attack the ovum ( secondary oocyte) . The corona cells become dispersed by enzymes released from the sperms and  from the mucosa of the tube. Only the capacitated sperm can pass freely through the corona radiata cells. Although  only one sperm fertilizes the ovum , it is thought that the other sperms aid the fertilizing sperm penetration through weakening of the protective barriers of the ovum.

Acrosome reaction :

It occurs when the sperm comes in contact with a zona protein in the zona pellucida, called  ZP3. During the acrosome reaction, the outer acrosomal membrane and the anterior head plasma membrane fuse together at many places , then the fused membranes are lost to produce multiple perforations , through which acrosomal enzymes are released. The acrosomal enzymes , including acrocin and trypsin-like substances , are essential to digest the zona pellucida and give access to the egg .  The membrane that covers the equatorial or posterior head region initiates fusion with the egg plasma membrane , but the inner acrosomal membrane does not participate in the membrane fusion and it is incorporated into the egg cytoplasm.

Phase II ( penetration of the zona pellucida ):

The zona protein responsible for both the sperm binding and  the acrosome reaction is the legand ZP3. More than one sperm try to penetrate the zona pellucida of the oocyte     , but only one needs to bind with the protein receptors in the zona pellucida to trigger an enzyme reaction allowing the zona to be pierced.   Only one sperm penetrates the zona pellucida , due to the  release of acrosomal enzymes (mainly acrocin), where  it comes in contact with vitelline membrane (plasma membrane) of the secondary  oocyte     . The contact between the head of the sperm and the surface of the oocyte stimulate the release of lysosomal enzymes from the oocyte. These enzymes induce a zonal reaction  that alter the properties of the zona pellucida to prevent further sperm penetration and inactivate sperm receptor sites on the zonal surface. Penetration of the zona pellucida takes about twenty minutes.

Phase III (fusion of the plasma membranes of  the sperm and  ovum):

The interaction between integrins on the oocyte and  the disintegrins on the sperm mediate , in part , the initial adhesion of the sperm to the oocyte.  The actual fusion occurs between the plasma membrane of the oocyte and  the plasma membrane covering the posterior region of the sperm head due to loss of the anterior part of the plasma membrane covering the acrosomal cap during the acrosomal reaction. Then , in humans , both the head and the tail of the sperm enter the oocyte  but the plasma membrane is left behind on the oocyte surface . Immediately after penetration of the sperm into the secondary oocyte, the zona pellucida and oocyte plasma membrane become impermeable to other sperms.  The secondary oocyte immediately finishes its secondary meiotic division and divides into the definitive oocyte (mature ovum) and , within 11 hours following fertilization, the oocyte has extruded the secondary polar body with its excess chromosomes.

The chromosomes of the ovum arrange themselves in a vesicular nucleus , called the female pronucleus. During the reactions of the ovum , the sperm is moving towards the female pronucleus  until  it lies close to it . The tail detaches and degenerates , and the  head  loosen and  swollen to form the male pronucleus . The male and female pronuclei are morphologically-indistinguishable and finally fuse togther . The fusion of the oocyte and sperm pronuclei nuclei marks the end of fertilization. The created  cell is called the fertilized ovum or the zygote which  is still surrouded by the zona pellucida. The zygote nucleus is called the segmentation nucleus  ,  as it immediately initiates a series of divisions known as segmentation or cleavage.

However, if the ovum is not fertilized , it loses its viability and  degenerates about 24 hours after ovulation and is discharged from the uterus in the debris of the next menstrual cycle. The sperms , if do not find the ovum, the lose their power of penetration and  fertilization after 48 hours of ejaculation.

Results of fertilization :

1.Restore the diploid number of chromosomes  ( 46 : 23 from the sperm and 23 from the ovum )

2.Determine the sex of the embryo ( this depends on the type of the sperm that fertilizes the ovum i.e. a sperm carrying Y-sex chromosome produces a male embryo (46,XY) and a sperm carrying X-sex chromosome produces a female embryo (46,XX).

3.Initiation of segmentation (or cleavage) of  the zygote to increase the number of its cells .